Metaplastic breast carcinomas (mBrCAs) are a subset of triple negative breast cancer (TNBC) that occurs with higher frequency in African and African-American (AA) women, have histological evidence of epithelial-to-mesenchymal transition (EMT), and poor prognosis compared with other TNBC. mBrCAs consisting of spindle cells are the most frequent and the most lethal subtype. In humans TP53 is mutated with similar frequency in mBrCAs and in non-metaplastic TNBC (60-80% of cases). At present, the defining molecular alterations of mBrCAs are far from understood, and the 5-year overall survival for patients with mBrCA is 54% compared to 73% for TNBC. Our lab has discovered that CCN6 protein is reduced in 68% of human mBrCAs compared to 33% of other breast cancer types (p<0.02). A major breakthrough in our lab during the previous cycle has been the generation of a mammary epithelial cell-specific Ccn6 knockout mouse model that demonstrates a tumor suppressor function for Ccn6 in mBrCAs. All mammary tumors in MMTV-Cre;Ccn6fl/fl mice resemble human spindle mBrCAs morphologically and at the transcriptional level, and they share increased nuclear localization of beta-catenin in 78% of tumors, and increased expression of the canonical Wnt target genes HMGA2 and IMP2 (IGF2BP2). Since the initial submission, we have discovered that extracellular CCN6 antagonizes the effect of Wnt ligands on beta-catenin activation in vivo and in vitro, but the mechanisms, cooperating events, and functional consequences need further investigation. Our CENTRAL HYPOTHESIS is that loss of CCN6 expression is required to drive spindle mBrCAs, at least in part by enhancing Wnt/beta-catenin mediated activation of pro-invasive and pro- metastatic targets, such as HMGA2 and IMP2, and that detection of CCN6, beta-catenin, HMGA2, and IMP2 proteins may serve as specific biomarkers of mBrCA in clinical tissue samples, with diagnostic and treatment utility. We propose three independent and complementary specific aims: AIM 1. To investigate the consequences of inducible mammary epithelial cell-specific Ccn6 knockout as a driver of the unique spindle mBrCA phenotype, and to investigate the cooperation with p53. AIM 2. To elucidate the molecular mechanism(s) by which CCN6 suppresses progression of spindle mBrCAs in vivo and in vitro. AIM 3. To evaluate the translational impact of CCN6, beta-catenin, HMGA2, and IMP2 in breast tissue samples of African, AA, and Whites. We have developed a unique mouse model and have characterized cohorts of human breast cancer tissues (n>4,000, including 200 from Ghanaian, and 275 cases of mBrCAs of all races) with clinical information and >15 years of follow-up. We have generated critical preliminary data, which provide a strong scientific premise. The reagents and expertise are in place in the PI and co- Investigator's laboratories. Our innovative studies are expected to provide insights into new diagnostic markers and therapeutic targets for this aggressive subtype of TNBC, which are currently nonexistent.